PORTLAND, Ore. — The number of Oregonians who have been infected with coronavirus may be ten times higher than testing would indicate, according to a new study headed by epidemiologists from the Oregon Health Authority.
The study was published on Friday in the CDC's Morbidity and Mortality Weekly Report (MMWR). The OHA staff who authored the study found that nine out of 897 blood specimens collected from 19 health care facilities throughout Oregon between May 11 and June 15 contained COVID-19 antibodies.
Findings also indicated that infection rates increased with age. No antibodies were detected in the blood of pediatric patients age 17 or younger. The rate was 0.4 to 0.5 percent in those ages 18-64, and 1.6 to 2.1 percent in those 65 and older.
“We suspected that a much larger segment of Oregon’s population has been exposed to and infected with COVID-19 than traditional diagnostic testing shows,” said Dr. Paul Cieslak, a study co-author and OHA medical director for communicable diseases and immunizations. “But these results also tell us that the great majority of Oregonians remain susceptible to this virus.”
Between Oregon's first confirmed case — identified at the end of February — and May 31, OHA said that 4,243 people in Oregon, roughly one out of every 1,000 people, had returned a positive result for COVID-19.
"Because this rate doesn’t account for people who were infected but did not seek testing — such as those with mild or no symptoms, or people unable to access testing — the rate is believed to be lower than the state’s actual infection count," OHA said.
OHA said that "seroprevalence" studies like this one may produce a more accurate rate of infection than conventional testing because it identifies people who have had previous infections but did not necessarily show symptoms.
“Because most of us are still susceptible,” said Cieslak, “we need to keep practicing physical distancing and masking until we have effective vaccines, treatments or other means of mitigating illness.”
Cieslak emphasized that antibody tests are not reliable if you suspect that you have COVID-19, because antibodies don’t appear in your blood until two to three weeks after exposure to the virus. Plus, he explained, “we don’t know if antibodies to COVID-19 mean you are protected, so you still need to wear a face covering, maintain physical distancing, and keep washing your hands.”